1. Field of the Invention
The present invention relates to a disk-type optical information recording medium capable of recording optical information, as well as to an optical information recording/reproducing apparatus which performs either one or both of recording and reproducing operations using the optical information recording medium. The invention also is concerned with a master-disk exposure apparatus for use in the production of the optical information recording medium.
2. Description of the Related Art
Hitherto, various systems have been proposed and used for optically recording and/or reproducing information.
The recording media employed in these systems are broadly grouped according to recording/reproducing method into three types: ROM (read-only), WOROM (overwrite) and R/W (rewritable). Each of these media employs a substrate made of a transparent material such as glass or polycarbonate, and various materials are applied or deposited on the substrate so as to provide functions of the above-mentioned three types. More specifically, a ROM type medium is obtained by depositing, on the substrate, a substance having high reflectivity and superior thermal stability, such as Al. A WOROM type medium is obtained when the material on the substrate exhibits capacity for irreversible reaction, such as, for example, organic colorants. A medium of R/W type is realized by using a material which magnetically or thermally exhibit reversible change, as is the case of a magnetic material or a phase-changeable material which can change from, for example, a crystalline state to an amorphous state and vice versa.
Optical information recording media also can be sorted according to configuration into disk-type media, card-type media and tape-type media. These types of optical information medium have their own advantages so that they are selected according to the use. Among these types of optical information recording media, disk-type media are most popular because of the speed of data transmission.
When a disk-type recording medium is used, data is recorded along a circumferential paths having predetermined lengths and such paths are referred to as a "data track" or simply as a "track". In case of a disk-type information recording medium, the data track can have the form of concentric circles or spiral form. From the view point of continuity of data transfer, however, it is preferred to use a spiral track, particularly when a large volume of data is to be handled.
FIG. 1 illustrates a conventional disk-type optical information recording medium having a spiral track.
Referring to FIG. 1, a disk 100 has guide grooves 103a to 103c spirally formed in the upper surface thereof, such that lands 102a and 102b are left between adjacent spiral grooves 103a to 103c.
In this optical information recording medium, the grooves or the inter-groove portions (lands) are used as data tracks along which a light beam for recording/reproducing information moves so as to record or reproduce information in and from the tracks, whereby a large volume of data can be handled without discontinuity. In recent years, a technique referred to as land/groove recording technique has been developed, in order to cope with the demand for greater data handling capacity. According to this technique, information is recorded both in the groove and on the lands, whereby the recording capacity is doubled.
FIG. 2 is a schematic illustration of a tracking servo system for use in an information recording/reproducing apparatus of the type which uses the optical information recording medium of FIG. 1 as a land/groove recording disk and capable of performing recording in accordance with the aforesaid groove/land recording technique.
Referring to FIG. 2, an optical head 122 associated with a disk 100 has an objective lens 122a, an optical system 122b, a light source 122c and a sensor 122d. The tracking servo system further includes an AT error signal generating circuit 123, a polarity change-over device 128, a phase compensator 129, a switch 130, a control circuit 131, an adder circuit 132, an actuator driver 133, and an actuator 134 which actuates the objective lens 122a so as to move this lens.
The disk 100 has lands and grooves formed in the surface thereof. Information is recorded in and reproduced from both the lands and the grooves.
The objective lens 122a is a pickup lens disposed to oppose the recording surface of the disk 100. A focus servo circuit and a focus actuator which are not shown are operative to control the position of the objective lens 122a such that light emitted from the objective lens 122a is constantly focused in the recording surface of the disk 100.
The light from the light source 122c is condensed through the optical system 122b and the objective lens 122a so as to form a spot of a predetermined diameter on the recording surface of the disk 100. The light reflected from the recording surface is again transmitted through the objective lens 122a so as to be converged on the sensor 122d which converts the light intensity into an electrical signal.
The AT error signal generating circuit 123 generates a tracking error signal based on the electrical signal output from the above-mentioned sensor. A known tracking error detecting method such as push-pull method, 3-beam method and so forth can be employed for the purpose of detecting any tracking error.
A polarity change-over device 128 performs change-over of later-mentioned polarity of the tracking error sinal generated by the AT error signal generating circuit 123. The polarity change-over device 128 is controlled by a control circuit 131 which also will be described later.
The phase compensator 129 performs phase compensation to stabilize the servo in response to the tracking error signal after change-over of the polarity performed by the polarity change-over device 128. The output line of the phase compensator 129 is connected to one of the input terminals of the adder circuit 132 through the switch 130 which operates under the control of the control circuit 131.
The adder circuit 132 has two input terminals to one of which the output line of the phase compensator 129 is connected through the switch 130 as stated above, while the other is connected to the output line of the control circuit 131 the output of which is delivered to the actuator driver 133.
The output signal from the adder circuit 132 is delivered to the actuator driver 133 which operates to convert this signal into an electric current signal in accordance with which the AT actuator 134 is driven to move the objective lens 122a.
The control circuit 131 includes a CPU which performs various controls such as control of turning on and off of the tracking servo and control of the movement of the objective lens 122a towards a target track, as well as the control of switching of the tracking error signal polarity performed by the polarity change-over device 128. The control of turning on and off of the tracking servo is effected by controlling turning on and off of the switch 130. The control of movement of the objective lens 122a towards a target track is performed by generating, while holding the switch 130 off, acceleration pulses for effecting shifting of the objective lens 122a towards the target track, and delivering such pulses to the adder 132. The change-over of polarity of the tracking error signal performed by the polarity change-over device 128 is conducted based on the position of the information track to be used for recording or reproduction, in accordance with the result of determination as to whether the track is a groove or a land.
A brief description will be given of the operation of the tracking servo system.
Light from a light source 122c is condensed through the optical system 122b and the objective lens 122a so as to be focused on the recording surface of the disk 100, thus forming a beam spot of a predetermined diameter. The light in the form of a spot is reflected so as to pass again through the objective lens 122a and is converged on the sensor 122d so as to be changed into an electrical signal. The electrical signal is delivered to the AT error signal generating circuit 123.
Upon receipt of an electrical signal from the sensor 122d, the AT error signal generating circuit 123 generates a tracking error signal based on the received electrical signal. FIG. 3 shows waveform of the tracking error signal generated by the AT error signal generating circuit 123 when the light spot is moved radially inward from an outer peripheral region across the lands and grooves on the recording medium of the type shown in FIG. 1, under such a condition that the focusing servo alone is operative, while the tracking servo is inoperative.
As will be understood from FIG. 3, when the light spot is moved radially inward across consecutive lands and grooves, the tracking error signal has the form of a sine wave which, for example, rises and crosses zero level at the moment at which the beam spot impinges upon the groove 103a and falls to cross the zero level again when the spot is on the center of the adjacent land 102a, the curve then rises and crosses the zero level when the spot passes the adjacent groove 103b. It is thus understood that the polarity of the tracking error signal is inverted depending on whether the beam spot is on the groove or on the land. It is therefore necessary to switch the polarity of the tracking error signal depending on whether a groove or a land is used for the recording/reproduction of information. In the illustrated arrangement, the switching of the tracking error signal is executed in the following manner.
The tracking error signal generated in the AT error signal generating circuit 123 is delivered to the polarity change-over device 128. The control circuit 131 discriminates, based on address information, whether the track to be used for the recording (or reproduction) is a land or a groove, and controls the polarity change-over operation of the polarity change-over device 128 in accordance with the result of the discrimination. As a consequence, the polarity change-over device 128 conducts the change-over of the tracking error signal, based on whether the track is a groove or a land.
The tracking error signal after the change-over of polarity performed by the polarity change-over device 128 is delivered to the phase compensation device 129 for phase compensation, and the resultant signal is delivered to the actuator driver 133 through the switch 130 and the adder circuit 132.
The actuator driver 133 converts the tracking error signal received from the phase compensator 129 into an electrical current signal which drives the AT actuator 134.
The apparatus is ready for recording (or reproduction) of information upon completion of the tracking servo control which is executed in the manner described above.
When recording or reproduction is to be executed on a different track, the switch 130 is turned off by the control circuit 131. The above-described tracking servo becomes inoperative due to turning off of the switch 130. The control circuit 131 generates acceleration pulses in accordance with which the objective lens 121 is moved towards the target track, and these pulses are delivered to the actuator driver 133 through the adder 132.
The actuator driver 133 drives the AT actuator 134 based on the acceleration pulses received from the control circuit 131. As a consequence, the objective lens 122a is moved to focus the target track.
Upon detecting that the movement of the objective lens 122a to the target track is completed, the control circuit 131 turns switch 130 on again, so that the above-described tracking servo control is commenced again.
The known optical information recording medium and information recording/reproducing apparatus using the same encounters the following problems.
Hitherto, in recording information on the optical information recording/reproducing apparatus in accordance with the land/groove recording technique, data to be recorded in the land portion and the data to be recorded in the groove portion are handled separately. Therefore, the volume of data handled continuously at one time is the same as that in conventional systems, although the recording or storage capacity has been increased to a value twice as large that in the conventional systems. It is thus impossible to meet the demand for higher operation speed of the system.
This problem will be discussed in more detail.
Recording of a large volume of data in the optical information recording medium of the type shown in FIG. 1 is conducted, by moving the objective lens 122a radially inward starting from the peripheral region or vice versa. For instance, steps are followed sequentially, such as turning off of the tracking servo, change-over of the tracking polarity, moving the objective lens to the land 102a, turning the tracking servo on, followed by recording of data, turning off of the tracking servo, switching of the polarity of tracking, moving the objective lens to focus the groove 103b, and turning the tracking servo on again, followed by recording of data. Thus, the continuity of the recording/reproducing operation is seriously impaired, because the groove/land polarity change-over operation and a track jump operation have to be executed each time the recording is finished with one track or over one full rotation of the disk.
An alternative way of recording is such that recording is performed without jumping operation such that data is recorded in the land 102a and then in the land 102b and, when the land portion has been fully occupied, the polarity of the tracking signal is changed so as to commence recording in the groove portion starting from the groove 103a, followed by recording in the groove 103b and so forth. In this case, however, the optical head is required to travel a large radial distance from the land at which the recording in the land portion terminates to the groove with which the recording in the groove portion is to be commenced, due to the large radial distance between these lands and grooves. Accordingly, this recording method fails to meet the requirement for higher speed of operation.